1. Technical Field
This invention relates to electrical imaging technology, and more specifically to an apparatus and method for enhancing the information obtained from conventional tomography, such as CAT and MRI scans, with accurate values of the electrical properties of the imaged object.
2. Background Art
Since Roentgen discovered the ability of x-rays to produce a shadowgram of the interior of a sample, there has been a substantial interest in scientific and engineering circles in technologies that allow the imaging of the interior of a sample using quantities measured exterior to the sample. While these technologies are often applied in industry and commerce generally, one of the most active areas for the use of imaging technologies is in the field of medicine. The original shadowgram x-rays were enhanced substantially with the discovery and perfection of computerized axial tomography, which allows the recovery of not just a shadowgram, but detailed information about the interior structure of a sample from x-ray intensities measured on the outside. A similar intense interest developed when nuclear magnetic resonance measurements were extended to map the interior of a sample in what is now commonly called Magnetic Resonance Imaging (MRI). Also, the use of ultrasound to explore the interior of samples has been a technology that has received substantial interest.
For all of these technologies (x-ray tomography, MRI, and ultrasound), an accurate picture in terms of spatial resolution is produced. However, information about the character of various objects located in the interior of a sample is often very limited. For example, x-ray tomography measures only the intensity of absorption at a single x-ray frequency and is generally simply proportional to the density of the material of the sample. The only improvements to conventional x-ray tomography have been to use x-rays at different frequencies which allow some information about both the density of objects in a sample (in terms of mass) as well as the electron density. Nonetheless, at best, only two new pieces of information are available from such measurements.
MRI is a modality that is sensitive to other parameters. Primarily, MRIs measure the number of protons (usually associated with hydrogen atoms) at any given point in a sample. Some extra information can be obtained with a great deal of analysis and care by measuring the decay time for certain magnetic resonance properties, but the sensitivity is such that perhaps three parameters can be measured using this technique.
A similar situation is obtained with ultrasound where there are yet some other complications due to the multiple reflections of the sound waves. While CAT scans and MRIs produce pictures that are somewhat familiar to even the untrained eye, ultrasound imaging requires a very skilled operator to perform the measurements and to interpret the results.
Because of the limitations of the existing imaging techniques, scientists and engineers have looked for other properties that might be exploited to produce an appropriate and improved image of the interior of a sample. One of these areas of exploration is in the use of electrical properties to produce such an improved image of the interior of a sample.
Therefore, there is a need for an apparatus and method for using the electrical properties of a sample to generate an improved image of the interior of the sample.
As a result of using the electrical properties of a sample to achieve an improved image of the sample""s interior, conventional electrical property imaging techniques have developed which measure the electrical properties of different materials located within the sample. Such conventional imaging has shown that substantial variations within a sample from one type of material to another (e.g., in a biological sample such as a human being, from one type of tissue to another) is a function of measuring frequency. Despite this achievement, the originally hoped for capitalization on these techniques has yet to occur. Conventional electrical property imaging suffers from a lack of accuracy, due to a sensitivity to noise, and a somewhat fuzzy pictorial resolution.
Such conventional electrical property imaging techniques are often referred to as xe2x80x9cimpedance tomography.xe2x80x9d Early work in impedance tomography was performed at the University of Wisconsin resulting in rough and inaccurate images. Subsequent impedance imaging groups were started in England, and in the United States at Rennsalaer Polytechnic Institute and Dartmouth College.
All of the conventional electrical property imaging techniques are based on the premises that 1) electrodes, or sensors, should be attached directly to the sample to be measured (for medical applications, the sample is a human body), and 2) specific current should be injected into the sample and the subsequent voltages measured. Therefore, these conventional imaging techniques implement a xe2x80x9cconstant current/measured voltagexe2x80x9d scheme. To date, the best result from such conventional techniques is approximately a ten to forty percent accuracy under typical measurement conditions.
In a departure from such conventional electrical property imaging techniques, the present inventor arranged sensors in an array outside the sample to be measured. See U.S. Pat. No. 4,493,039. Further, during imaging of a sample, the voltage was held constant while the current was measured. This method of using a xe2x80x9ccontrolled voltagexe2x80x9d was opposite of the current teachings in the relevant arts. Despite this new technique, it was impossible to generate accurate images of the interior of a sample because the sensors did not have any contact with the sample being measured. A further limitation to this controlled voltage method was the uniqueness of the imaging mathematics which was not fully realized at that time. Therefore, at the present time, there is a need for an apparatus and method that performs electrical property imaging using controlled voltage and in which sensors contact a sample being measured. There is still a further need for such an apparatus and method that takes full advantage of the uniqueness of the imaging mathematics associated with such controlled voltage imaging techniques.
Due to the high number and cost of existing imaging devices (CAT scans, MRIs, and ultrasounds), there is also a need for an apparatus and method that enhances the use of such existing imaging devices by adding information regarding the electrical properties of each region that is imaged in the interior of a sample.
The present invention solves the problems associated with conventional electrical imaging techniques by providing an Electrical Property Enhanced Tomography (EPET) apparatus and method that generates an accurate calculation of the electrical properties of a sample""s interior subregions. The EPET apparatus is a plurality of sensors arrayed on a sample holder, in which a sample is place, that measure the net total charges Q on the surface of the sample holder.
Specifically, the EPET apparatus fixes the external voltages along points on the sample holder to generate an electromagnetic field within the sample holder. The EPET apparatus measures the electrical currents at a number of those fixed external voltage points on the sample holder, wherein the electrical currents are related to the electrical current that the electrical properties of the sample allow to pass according to the electromagnetic field.
The EPET apparatus of the present invention is connected to a tomographic device, e.g., a CAT scan or MRI device, for generating additional information on the electrical properties, i.e., the dielectric constant and conductivity, of a sample being measured. The EPET apparatus comprises one or more capacitive sensor arrays, each of which comprise a segmented capacitive sensor plate having a plurality of controller units connected thereto. Each controller unit controls the voltage and frequency in order to produce an electromagnetic field in the sample holder and then detects the net total charges Q on each segment of the capacitive sensor plate (which is related to the net total charges q within the sample when placed within the electromagnetic field of the sample holder). The EPET apparatus then uses the net total charges Q to calculate enhanced values of the electrical properties (the dielectric constant and conductivity) of the sample by using electromagnetic mathematical theory. In addition, a matching medium is applied to the sample during operation of the present invention such that the matching medium fills up the entire closed space and contacts the capacitive sensor arrays. In the preferred embodiment, the matching medium is chosen with dielectric constant and conductivity values that maximize the accuracy of the measurement.
An advantage of the present invention is that it enhances existing tomographic devices, e.g. CAT scans or MRIs. Therefore, current health care facilities can continue to use their existing devices, but with greater accuracy in imaging.
Another advantage of the present invention is that the processing is inherently parallel. Therefore, the imaging technique of an EPET apparatus and method is very fast as well as precise and robust.